In the mass-production of consumer products it is often necessary to print product information and the like on the objects as they move along a conveyor. The printing has to be carried out quickly and without disrupting the continuous or step-wise advance of the objects along the conveyor path. The method for printing on objects that is current in industry uses silk-screen printing, tampon printing, offset printing, flexoprinting, ink-jet printing or similar processes. Thus, any objects such as, e.g. bottles, cans or even data carriers, especially digital-data disks such as CD's and DVD's are entirely or partially printed preferably with silk-screen processes since serigraphs have properties in part that can not be produced or produced only with difficulty with other printing processes such as, e.g. flexoprinting or offset printing.
A type of silk-screen printing that is frequently used when printing rather small objects is the flat silk-screen printing process. Flat silk-screen printing mechanisms use a screen that is flatly clamped in a frame and contains the information to be printed. Such printing mechanisms operate, based on principle, in such a manner that the surface of an object to be printed stands still relative to the printing screen during the printing. The printing process thus takes place in such a manner that printing ink is pressed through the printing screen with a silk-screen squeegee. The printing screen contains the printing image to be transferred here in such a manner that the printing screening is permeable for the printing ink only at the areas to be printed and therefore the printing ink can be transferred to the object to be printed only at these locations.
In addition to a printing machine, a lifting and lowering devices for the silk-screen printing frame or an object holder are required in addition to the cited silk-screen printing frame and a corresponding holder for it in order to be able to raise the object to be printed during the printing close enough to the screen frame, and on the other hand to ensure a sufficient amount of distance to the printing screen while the object is moved toward and away from the printing station.
Furthermore, moving elements for flood squeegees and printing squeegees are required that ink the printing screen with ink and transfer the printing image onto the object. All these elements must be more or less rapidly mechanically moved and in addition positioned with great accuracy so that limits are set to an increase in the speed of the mechanical operating sequences for greater productivity, and these limits can only be overcome with significant technological complexity. This applies in particular if objects are transferred in steps through a printing station during which an object is transported with each step and a printing must take place in the time between two steps. If the throughput of objects is to be increased in such an instance, this has an immediate negative influence on the time available for the printing.
In U.S. Pat. No. 7,171,896, for example, the printing screen and with it even part of the screen-printing mechanism are moved synchronously with the continuously moving object to be printed. As a result, the object does not have to be stopped for the printing but rather can be printed during a continuous and synchronized movement so that the time of the start-stop phases for stopping the object under the silk-screen printing mechanism and removing it again after the printing has taken place can be saved.
This has the disadvantage that the printing screen and the ink in it are exposed to high acceleration forces since, after a printing operation, the printing screen with the ink in it must be moved back as rapidly as possible into the starting position for the following job. Since the printing ink is thrown back and forth during the above, problems with a homogeneous distribution of ink on the printing screen and thus in the printed image can result, and there is furthermore the danger of ink being spilled out of the silk-screen printing frame, which can result in contamination of the machine or of the objects to be printed.